Photograph 1

Corrosion of Aluminum Flashing in Contact with Pressure Treated Wood Decks

DonanEngineering Articles, Forensic Engineer, Forensic Engineering, Structural Damage & Collapse

When decks are attached to a house, flashing is installed under the siding and over the deck joist attached to the rim joist of the house to direct water away from the joists.

The deck joist is normally pressure treated wood and the rim joist is not normally pressure treated wood.  If the flashing fails the rim joist is exposed to water and will eventually rot.  The rot and decay can occur undetected for an extended time, because the deck joist covers it from view on the exterior and a basement ceiling or insulation covers it from view on the interior.  The problem is usually discovered when unexplained debris is found on the basement floor or water stains are observed on the finished basement ceiling.  Removing the ceiling and/ or the insulation reveals a rotted rim joist (Photograph 1).  Rot can extend into the ends of floor joists and into the subfloor under carpet or hardwood floors.

Removal of the deck boards reveals corroded aluminum flashing (Photograph 2).   Corrosion occurs where the flashing is in contact with the pressure treated deck joist and pressure treated deck boards.  Water seeps into the house because the aluminum flashing which was installed to prevent water intrusion is corroded resulting in holes in the flashing.

Photograph 2

Photograph 2

The water intrusion then causes the rim joist and subfloor to rot and decay.  Wood rot occurs when wood is repeatedly exposed to moisture and not allowed to dry out.  Rot is the result of long-term exposure to moisture, indicating that the damages observed are not the result of a one-time weather event.

Water seeps into the house because the aluminum flashing which was installed to prevent water intrusion is corroded resulting in holes in the flashing (Photograph 3).  Galvanic corrosion is a slow process and may take five to seven years to create holes large enough to allow significant water intrusion.  Water intrusion then causes the rim joist and subfloor to rot and decay.

Photograph 3

Photograph 3

The aluminum flashing corrodes because of contact to wet, pressure-treated lumber.  The chemical used to treat the wood is chromate copper arsenate (CCA).  The copper in the chemical reacts with the aluminum in a galvanic reaction that corrodes the aluminum.  The corrosion is accelerated in the presence of moisture.  When the deck boards are laid tight, water is trapped on the flashing and the area rarely dries out.

CCA treated lumber production was phased out in 2003-2004, primarily because of the arsenic in the chemical.  The chemicals used in newer treated lumber are even higher in copper and because of the likelihood of more aggressive corrosion, aluminum is not recommended for flashing in contact with pressure-treated lumber.  Currently copper, stainless steel, or non-metallic flashings are recommended to prevent galvanic corrosion.